All works great except I can't fool Silverstripe into producing the correct links by itself ; it always produces links to the real location instead of the location I want. That has to do with SS using the wrong Base URL ....
If I try to manually set my base URL to "/" or "www.mydomain.com" , it throws the error I mentioned in the post above..
In the site itself I can manually fix this by building links by concatenating a "/" and
$URLSegment , but the sitemap.xml is all screwed up.